Position of simplesects

I do not understand the following:

It is valid DocBook to use a simplesect like this:

<sect2>
   <title>...</title>
   <sect3>...</sect3>
   <simplesect>...</simplesect>
</sect2>

but NOT, when the sect2 starts with introductory <para>s

<sect2>
   <title>...</title>
   <para>bla bla bla</para>
   <sect3>...</sect3>
   <simplesect>...</simplesect>
</sect2>

At least I get a validation error when I validate the 2nd example.
Is this a bug or a feature? I can't figure out the precise rules where 
exactly I can use a simplesect.

Joachim

-- 
Joachim Ziegler                       Stuhlsatzenhausweg 85
Max-Planck-Institut für Informatik    66123 Saarbrücken, Germany
Email:           Tel.: (+49) 0681 9325-127